(1) Field of the Invention
The invention relates to a linear actuator comprising a brushless multiphase electric motor, including a stator and a rotor, the latter acting on a control organ through driving means designed capable of converting, over several revolutions, its rotational movement into a linear displacement.
The present invention relates to the field of the linear actuators generally including a brushless multiphase electromagnetic motor. It finds a very particular application in the case in which a control in the form of a fast linear displacement is sought, as is necessary, for example, for controlling the valve of a device for re-circulating the exhaust gases of a diesel motor, but also for controlling air-inlet valves.
(2) Description of the Prior Art
Nowadays, for such applications use is made of direct-drive linear electromagnetic actuators or linear actuators based on an electric stepping motor using a system for converting the rotational movement into a linear displacement, such systems being capable of adopting various embodiments. In particular, cam systems, pinion and rack systems and screw and nut systems are known.
From DE-A-100 03 129 is known in particularly a linear actuator including a stepping motor provided with a rotor provided, on its periphery, with magnets of alternate polarity in front of pole shoes of a stator. It includes at least two electric field coils allowing to control the motor through electronic switching. It should be noted that in the axial extension of the stator is provided for a Hall sensor surrounding the rotor as a position-detection device.
Turning back to the rotor, it includes, internally and coaxially, a tapped nut engaging a threaded rod immobilized in rotation.
Thus, from the action of the rotation of the rotor and, hence, of the nut integral with the latter, results a displacement in translation of the threaded rod which substantially forms the control organ.
The problem raised by this kind of linear actuator with a multiphase motor consisted in that, in the event of failure of the motor, even if due to an interruption of power supply, the control organ and, hence, the part, for example the valve on which it acts, remains in the position reached before the failure occurs. Therefore, since it does not return into a safety position, this can result into a more serious dysfunction at the level of the unit in which this controlled part fits.
When taking, for example, the particular case of the control of valves of a device for re-circulating exhaust gases of a diesel motor, it is imperative that these valves be maintained closed on their seat, so as to prevent the exhaust gases from being re-circulated when such a failure occurs, for otherwise the operating conditions of the motor itself will be altered.
Therefore and as described in U.S. Pat. No. 4,501,981, there has been devised to provide this kind of actuator with a stepping motor with springy restoring means capable of restoring the threaded rod into a reference position in the event of power fail. Though these springy restoring means can adopt the shape of a helical spring acting directly on the threaded rod, in a second embodiment described in this document U.S. Pat. No. 4,501,981 a helical spring can also act on the rotor in order, in the event of such a power fail, to control the later in rotation and to restore the control organ into its reference position.
However, it should be noted that a D.C. multiphase stepping motor raises a problem of response time and jerked displacement since a magnetized pole of the rotor has a privileged balanced position when it is placed in front of a pole of the stator or when a transition between two magnetic poles is located in front of such a stator pole.
The residual torque is thus a periodic function of the angular position the frequency of which depends on the number of magnetic poles and on the number of stator poles.
Finally, the stepping motor has two kinds of significant drawbacks for both ensuring a fast control of an organ and allowing the latter to be easily restored into a reference position under the action of a springy restoring:                the residual torque, which corresponds to the torque without current of the motor, is excessive and prevents an easy restoring into a reference position,        the principle of operation of the stepping motor only allows controlling a displacement, without having the possibility of checking whether the imposed sequence has been carried out correctly.        
In this respect, in FR-A-2,754,953 has described a brushless and electronically switched multiphase motor having a low residual torque. In particular, the stator portion of this motor has at least two W-shaped circuits including, each, an electric coil surrounding the central stator pole. These W-shaped circuits are so arranged that, when one central stator pole is located in front of a magnetic transition, the other central stator pole is located roughly in front of a magnetic pole. The pole shoes of these central stator poles of the two W-shaped circuits belong to different phases and are angularly separated by 120°. Thus, the shape of the W-shaped stator circuit ensures the closing of the field lines between the central pole which receives the coil and both adjacent poles.